Small computer system interface (SCSI) is a set of standard electronic interfaces that allow personal computers to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers scanners faster and more flexibly than previous interfaces. Conventional SCSIs employ parallel interfaces. SCSI ports continue to be built into many personal computers today and are supported by all major operating systems.
In addition to faster data rates, SCSI is more flexible than earlier parallel data transfer interfaces. The SCSI standard can transfer data at up to 80 megabytes per second (MBps). SCSI allows up to 7 or 15 devices (depending on bus width) to be connected to a single SCSI port in a daisy chain fashion. This allows one circuit board or card to accommodate all the peripherals rather than having a separate card for each device, making it an ideal interface for use with portable and notebook computers. A single host adapter in the form of a PC card can serve as a SCSI interface for a laptop, freeing up the parallel and serial ports for use with an external modem and printer while allowing other devices to be used in addition.
A widely implemented SCSI standard uses a 40 MHZ clock rate to raise data transfer rates up to 80 MBps. It provides a longer possible cabling distance (up to 12 meters) by using low voltage differential (LVD) signaling. Earlier forms of SCSI use a single wire that ends in a terminator with a ground, called single ended (SE) signaling. LVD sends the signal over two wires with the data represented as the difference in voltage between the two wires. This allows support for longer cables. Low voltage differential reduces power requirements and manufacturing costs. High voltage differential (HVD) is another SCSI standard that has been widely implemented. Conventionally, SCSI systems have been designed to employ these standards either individually or in combination. However, more recently the LVD and SE standards have gained in popularity while many SCSI devices such as SCSI controllers do not support the HVD standard.
SCSI systems that employ a plurality of SCSI devices to communicate with a single peripheral device have encountered substantial problems. One problem is the inability to easily disconnect a specific channel of one SCSI controller without affecting other SCSI controllers connected to a SCSI bus, or the other SCSI channel resident on the same SCSI controller. Using conventional methodologies an entire controller must be disconnected in order to disconnect a specific channel of a SCSI controller. As is suggested, this is disadvantageous for dual channel SCSI controllers and involves the physical removal of the SCSI controller containing the specific channel desired disconnected.